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pressure tank questions

I have one or two problems--I hope it is just one.

Lately, the garden hose sprinkler will reduce to a dribble after about half an hour. The pressure tank is still cycling; it builds up to 70 pounds on the gauge, and appears to be sending, but the sprinkler starts just dribbling; if I shut the spigot and then open it again after an hour, the sprinkler will work again, for a while.

Also there is a black steel part on my pressure tank that spits and sputters while the garden hose or sprinkler is on. This did not just begin; it has been doing this for a couple of years, long before the sprinkler developed this new problem. The line to the pressure gauge does not come out of this part. There is no other regulator or relief valve that I can detect.

Can someone tell me if I replacing the black part should fix both problems? And how to replace this part. This is an old steel tank system, probably 20 years old, with no Schrader valve that I can see. Should I be adding or removing air to the pressure tank, and if so how?

OK, I have learned that this is an AVC or air volume control. Apparently it has an adjustable float inside the tank. I assume it is bad, because of the sputtering, but maybe that is just an adjustment issue. I find these AVCs online only a couple of places. Now I am wondering if I have to buy a whole new tank. Advice anyone??